In an optical transmission system that uses wavelength multiplexing, the standardization of OTU (Optical-channel Transport Unit) 4 was completed as a technique used for transmitting a client signal of about 100 Gbit/s per one channel. In addition, for example, in order to realize the long-distance transmission of a signal having a transmission rate of 100 Gbit/s, a modulation method such as Quadrature Phase Shift Keying (QPSK) or the like has been put to practical use. In addition, in order to further increase transmission capacity, a modulation method has been proposed in which a multilevel modulation method such as 16 Quadrature Amplitude Modulation (QAM) or the like, the number of the levels of which is greater than 4, or Orthogonal Frequency Division Multiplexing (OFDM) is used.
However, many of optical transmission devices used in an optical transmission system of the related art constantly operate at maximum transmission rates, regardless of the states of traffic loads or the transmission rates of the physical interfaces of client devices. In this case, since the optical transmission device operates in response to the maximum transmission rate, a stuffing byte, an idle signal, or the like may be transmitted in addition to a data signal. Namely, the transmission efficiency of an optical transmission device of the related art may be reduced, depending on the transmission rate of a client signal. In addition, in this case, since an unnecessary signal is transmitted, useless electric power turns out to be consumed.
As a related technique, there is the following modulation method. Namely, an intersignal distance that is a distance between positions arranged of modulated signals is widened for data having low noise tolerance, and the intersignal distance is narrowed for data having high noise tolerance. For example, in the signal point arrangement of 8PSK, a ratio between the transmission rate of a data bit sequence and the transmission rate of a coded voice information bit sequence is set to 2:1, high 2 bits are allocated to data, and low 1 bit is allocated to a voice signal sequence. In addition, when data communication is not performed, a modulation method is switched to BPSK (Binary Phase Shift Keying), and voice communication is performed. On the contrary, when voice communication is not performed, a modulation method is switched to π/4 shift QPSK, for example, and data communication is performed. The detection of the switching of a modulation method, which is to be performed on the receiving side, is performed in accordance with switching control information accommodated in a unique word that is located at the front of a frame and modulated with BPSK. An example of the technique is disclosed in Japanese Laid-open patent Publication No. 2004-201131.
In addition, as another related technique, there is a data transmission device in which important data is transmitted and received through a PSK modulator and a PSK demodulator, the state numbers of which are small, and unimportant data is transmitted and received through a PSK modulator and a PSK demodulator, the state numbers of which are large. An example of the technique is disclosed in Japanese Laid-open patent Publication No. 2-278940.